1. Field of the Invention
The present invention generally relates to the field of maskless lithography (MLL) and optical maskless lithography (OML).
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate or part of a substrate. A lithographic apparatus can be used, for example, in the manufacture of flat panel displays, integrated circuits (ICs) and other devices involving fine structures. In a conventional apparatus, a patterning device, which can be referred to as a mask or a reticle, can be used to generate a circuit pattern corresponding to an individual layer of an integrated circuit, flat panel display, or other device. This pattern can be transferred onto all or part of the substrate (e.g., a glass plate, wafer, etc.), by imaging onto a layer of radiation-sensitive material (e.g., resist) provided on the substrate. Instead of a circuit pattern, the patterning device can be used to generate other patterns, for example a color filter pattern or a matrix of dots.
Instead of a mask, the patterning device can comprise a patterning array that comprises an array of individually controllable elements (e.g., pixels of the patterning device). The pattern can be changed more quickly and for less cost in such a system compared to a mask-based system.
A flat panel display substrate is typically rectangular in shape. A substrate for integrated circuit application is typically circular in shape. Lithographic apparatuses designed to expose substrate of these types can provide an exposure region that covers a full width of the substrate, or covers a portion of the width (for example half of the width). The substrate can be scanned underneath the exposure region, while the mask or reticle is synchronously scanned through a beam. In this way, the pattern is transferred to the substrate. If the exposure region covers the full width of the substrate then exposure can be completed with a single scan. If the exposure region covers, for example, a small portion of the width of the substrate, then the substrate can be moved transversely after the first scan, and further scans are typically performed to expose the remainder of the substrate.
Optical rasterization is a technique that uses a description of a desired pattern to be printed (e.g., a graphic design system (GDSII mask file)), to compute states (e.g., pixel transmittance or pixel micro-mirror tilt or piston) of the patterning device pixels that will reproduce the pattern at an optical image plane. In maskless lithography, rasterization is a technique used to form pattern data, which is used by a pattern controller to control the patterning device. For example, controlling the patterning device can include moving individual controllable elements that are associated with the patterning device, e.g., mirrors.
In various examples, rasterization is performed by taking a desired pattern and Fourier transforming the pattern to determine what the pattern would look like in the pupil or the image plane of projection optics. Pattern data correlated to this determination is generated and transmitted along a datapath to the pattern controller. An algorithm matches performance of the patterning device to the pattern data to produce control signals that are used to form a pattern on the patterning device to pattern incoming illumination. The projection system directs the patterned light to reproduce the desired image at the pupil of the projection system.
However, rasterization is usually performed assuming, the projection system includes perfect optical elements. Unfortunately, in most projection systems, the optical elements contain minor imperfections, which can produce certain aberrations and/or distortions in the projection system. These aberrations and/or distortions can effect the actual image formed in the projection system, sometimes enough to cause errors in the features formed on the substrate. In addition, rasterization generally assumes that the illumination light reaching the pattern generation system is completely uniform, and any illumination profile is perfectly defined. Unfortunately, most illumination systems contain minor imperfections that can cause the beam to be non-uniform or the illumination profile to be slightly misshaped, resulting in errors in the features formed on the substrate.
Therefore, what is needed is a system and method that compensate for aberrations and/or distortions in an image formed in a projection system, as well as illumination pupil fill and field uniformity imperfections, by accounting for such imperfections by altering the pattern data generated to control a patterning device.